Anyway I had to remove it from my computer because of its incompatibility with my favorite file manager (Double Commander aka. doublecmd-gtk).
I don't understand what makes a theme work with it or not but I know that Starlabs and Adwaita-dark are working fine with this app. How? no idea, sorry to be of no help.
I will be glad to use it again if it becomes compatible with that particular app.

Just for reference if another user has the same issue one day:
I don't remember exactly why but I kept the habit to install custom themes to ~/.local/share/themes in my old Ubuntu 16.04. But I have a backup of my old system and while browsing it today I see that I had created a ~/.themes symlink pointing to ~/.local/share/themes. Im'not sure but even then, I may have done this trick to fix the DC theme.

So now I've tried to copy your theme's directory to ~/.themes on my current system, and... the magic happened ! I've now moved all my installed themes to this directory instead of my unclear symlink trick, so the apps already impacted by these themes before the change are still properly styled, and the only app to ignore the system theme is now as beautiful as the rest.

The solution was so simple. And the cause was just... my own mistake. Apologies. Happy end. And thanks again for your work on this theme ! :)